• Multifactorial characters: many factors, such as genes and environment affect phenotype
  • Pedigree Analysis: Square is male, circle is female, painted in means they have that trait
    • X-linked dom → father passes to all daughters; X-linked recessive → mother passes to all sons

Relationship Among Alleles of a Single Gene                                                                                     

Complete dominance: Heterozygous phenotype is the same as the

  1. dominant
  2. Incomplete dominance of either allele: heterozygous phenotype is intermediate between two homozygous
  • Dominant allele doesn’t make that much protein, recessive makes none, so heterozygous makes weaker amount
  1. Codominance: both inherited alleles are completely expressed in heterozygotes                                                                   
  • People that are MM (L^M, L^M) produce the one molecule that appears on surface of blood cells, NN (L^NL^N) produce the other; and those who are MN (L^M, L^N) produce both
  1. Pleiotropy: one gene affects multiple phenotypic characters
  • Ex: sickle cell disease
  1. Multiple alleles: some genes have more that two alleles
  • Ex: blood group that produces A, B, and O blood types, there are 3 possible alleles → I^A, I^B, or i
    • Superscripts used because the two alleles, A and B, are codominant; lowercase i is recessive when expressed with others
    • There are six possible genotypes representing all possible combinations of 2 alleles: [I^A, I^A] and[ I^A, i] (A blood type), [I^B, I^B and I^B, i] (B blood type), [I^A, I^B] (AB blood type) and [ii] (O blood type)
    • 4 phenotypes correspond to presence/absence of an A or B sugar component attached to plasma membrane of red blood cells (A sugar, B sugar, I^A, I^B both sugar, and ii no sugar)

Relationships Among Multiple Genes

  1. Epistasis: Expression of one gene affects/masks another
  • Ex: hair color in labradors where B codes for melanin better (black) than b (brown). Second allele E needed to deposit melanin; ee is dysfunctional so no produced melanin is deposited (yellow)
  1. Polygenic Inheritance:
  • Interaction of many genes that affect a single phenotype (Ex: height, very short → very tall)
  • Quantitative Characters: Controlled multiple gene which vary/add up along a continuum; affected by polygenic inheritance
    • g. eye color, skin color → three genes produce melanin, skin color determined by how much genes are expressed
    • Any human character that is polygenic, cannot be predicted (like eye color)

Environmental Impact on Phenotype

  • Environmental factors influence gene expression and can lead to phenotypic plasticity.
    • Phenotypic plasticity: When the same genotype can result in multiple phenotypes under different environmental conditions
  • Examples:
    • At high temp & pH enzymes & transcription factors become denatured
    • Environment might contain a molecule that acts as a repressor or activator

Case Study: Identical Twins

  • Although have same DNA, expression of DNA influenced by environmental factors
  • Before are born → experience slightly different environments in uterus → affect gene
  • expression → different phenotypic expressions


  1. Nutrition: not enough nutrients can inhibit growth and plants without enough nitrogen may not flower
  • May also influence expression of genetic disorders (lactose intolerance)
    • Ex: ppl cannot metabolize specific amino acid → amino acid accumulates → brain cells die → death; minimizing amino acid → safe
    • Organisms w/ mutation so cant syhtnesize amino acid can grow in environment w/ amino acid
  1. Temperature:
  • Influences sex dertermination in some reptiles
    • Eggs incubated at lower temperatures become males; those at higher temperatures become females
  • Influences fur & skin color of animals, melanin production (more UV = more melanin)
  1. Soil pH:
  • Influences flower color; blue in low pH and pink in high pH
  1. Released Chemicals
  • Chemical signals can affect gene expression and often needed to elicit mating
    • Ex: bacteria secrete signaling molecules that stimulate nearby bacteria to aggregate & form biofilms
    • Ex: yeast cells only mate with yeast cells of opposite mating type; yeast releases signalling molecule (pheromone) → only opposite yeast respond

Multifactorial Diseases and Disorders

Behavior of Recessive Alleles

  • Recessive allele that causes a genetic disorder (a) codes for no protein or malfunction or misfolding
  • Recessively inherited: must be homozygous recessive
    • Carriers: heterozygous individual with recessive allele and can pass on disease
      • (Aa) usually have normal phenotype because (A) codes for enough
    • Cystic Fibrosis: caused by a mutated channel gene that causes ppl with two recessive to not have chloride transport
      • Pleiotropy: affects multiple organs
    • Sickle Cell Anemia: mutated hemoglobin gene also recessively inherited
    • Tay Sachs Disease: mutated gene codes for a defective lipid breakdown protein in the brain
      • BUT at the molecular level, is incomplete dominance, bcuz only digest half the amount of enzymes

Dominantly Inherited Disorders

  • Lethal Dominant: only needs one copy to kill (heteroz)